The present invention relates to a process for preparing high-purity aluminum oxide and to the use of the aluminum oxide thus obtained as defined in each of the claims.
Aluminum oxide (Al2O3) per se is known.
High-purity aluminum oxide is required inter alia for ion-conducting ceramics. For this purpose, the content of extraneous metals, more particularly of calcium, must not be greater than 50 ppm. Preference is given to even smaller calcium contents.
Ion-conducting ceramics are described, for example, in J. L. Sudworth and A. R. Tilley, The Sodium Sulphur Battery, Chapman and Hall, New York (1985) and are used inter alia as an electrolyte in electrochemical cells, for example as electrical batteries or synthesis cells for alkali metals. In simplified terms, ion-conducting ceramics are generally produced as follows: aluminum oxide, an alkali metal source, generally alkali metal salts, and further additives are shaped (green bodies) and sintered at very high temperature.
Pure aluminum oxide is commercially available but is frequently so expensive that it is prohibitive for industrial use.
JP 5-17043 (SUMITOMO) describes a process for preparing an aluminum oxide with a low CaO content, by washing aluminum oxide which has been obtained by a Bayer process and in which the CaO content is 120 ppm or less with an organic acid, and filtering. A frequent occurrence here is that the fine aluminum oxide is present floating in the suspension and can only be filtered very slowly, which is disadvantageous in an industrial process.
U.S. Pat. No. 3,104,944 describes a process for preparing aluminum oxide, wherein an aluminum salt comprising an ionic alkaline earth metal impurity is converted at an acidic pH of 4.5 to 7 to an aluminum hydroxide hydrogel, and the alkaline metal is extracted with an amino-substituted carboxylic acid, and the hydrogel is washed and calcined.